how can I stop freestyle from drawing in mist?
You can avoid freestyle rendering in mist by setting a "distance from camera" modifier. 


Goto "Render Layers"
Scroll down to "Freestyle Line Style" Panel
Click "Alpha"
Click "Add Modifier" and then select "Distance From Camera"
Select "Mix" for the blend type
Select "Curve" for mapping type
Add a point in the center of the curve ui
set the right most point x pos to 0.3 and y pos to 0.0
set the center point x pos to 0.25 and y pos to 0.5
set the left most point x pos to 0.0 and y pos to 1.0


EDIT:


set the Range Max to the depth of your mist.




PROTIP: Gain more control over falloff by adding points to your curve. Adjust shape until the "Distance From Camera" modifier falloff matches your mist.